Combined internal and external swing up toolholder for threading.
Swing up toolholders for threading are a very convenient and offer a much simplified procedure. Gone are all the laborious stages of retracting and advancing the tool after each pass and instead each cut is made, the lathe is then reversed, whilst it is reversing the next increment is added, and then the next cut is made. This is much less laborious and much less prone to error.
This toolholder is a combination of the elements of my swing up toolholder for external threading together with the elements of the newly developed swing up toolholder for internal threading. It is designed to fit on my quick change toolpost.
The photo above shows the tool holder with the external threading side facing the headstock. This is identical to the external swing up threading toolholder previously described in most respects. It differs in three aspects. Firstly the bracket supporting the swing up tool clamp is now screwed directly to the toolholder block. Secondly, the pivot screw has now been replaced by a pivot rod that passes all the way through the the tooholder block. Finally the tool clamp is extended slightly to allow a grub screw to clamp the swing up tool clamp to the pivot rod. The grub screw can be seen emerging from the back of the tool clamp.
This photo shows the toolholder turned through 180 degrees so that the swing up internal threading tool is towards the headstock. This is the same as the prototype internal threading tool previously described with a few minor modifications. The external swing up tool clamp has replaced the piece of steel used as a weight. The M4 socket head screw on the face of the block has been added to provide a positive stop. On the prototype version the stop was attached to the weight and it acted through the whole length of the pivot rod which introduces some torsional flexibility. The new stop screw eliminates this. The final modification is that a height adjusting screw has been added. This is the knurled screw sticking out of the top of the block. This can raise or lower the block on the toolpost. Note the pivot rod just below tool. The tool and the pivot rod are clamped by M4 grubscrews in the two tapped holes on the front face of the small block
When the external toolclamp is swung back the two countersuck screws securing the bracket to the block can be seen. Also visible is the locating pin attached to the underside of the tool clamp. With the tool swung back like this it is easy to inspect and make measurements on the work piece
The bracket attached to the toolpost has a short slot cut in the end. The locating pin slides into this slot to prevent any lateral movement of the tool clamp.
This shows the internal threading tool swung back.
This one toolholder thus provides all the facilities of the two toolholders it replaces. The internal toolholder is currently fitted with an 8 mm diameter tool allowing it to be used for threading holes as small as 10 mm diameter. By employing a smaller tool even smaller threaded holes should be possible. However most holes smaller than 10 mm in diameter can usually be more conveniently threaded using a tap.
Initial tests of internal threading made with the tool were in some scrap aluminium tube. The tool performed well and nice clean threads were cut. However, when the tool was later tested on steel there were problems with chatter and the end result was not good. There were no problems with the external threading arrangement.
The reasons for the problems were obvious when cutting steel since the tool deflected considerably as soon as the cutter hit the metal. Examination showed that this lack of rigidity was due to:
1. Significant movement of the tool in its mounting hole.
2. Significant movement where the pivot shaft entered the swinging arm.
3. Some flex of the pivot rod itself.
A new, much sturdier tool has been constructed, see here.